1. Field of the Invention
The present invention relates to exhaust gas purifying apparatus and method for an internal combustion engine and particularly relates to a technique of abnormality determination of an NOx removing (or reduction) catalyst to purify (or reduce (remove)) nitrogen oxides (NOx) in an exhaust gas of the engine.
2. Description of the Related Art
In the internal combustion engine mounted in an automotive vehicle, especially in the internal combustion engine in which a fuel mixture air under an oxygen excessive state is combustible (drivable under a air-fuel ratio of a lean state), an exhausted quantity of NOx (nitrogen oxides) during a lean drive is increased. Hence, a technique of preventing NOx exhausted from the engine with an NOx reduction (removing) catalyst disposed in an exhaust passage of the engine from being released toward the air. The r NOx reduction (removing) catalyst absorbs NOx in the exhaust gas when an air-fuel ratio of the exhaust gas streaming into the NOx removing catalyst is high (namely, lean) and the absorbed NOx in the NOx reduction (removing) catalyst is released while NOx is reduced into (N2, nitrogen) when the air-fuel ratio of the exhaust gas streaming into the NOx reduction catalyst is low and a reducing agent (HC (Hydro Carbon), CO (Carbon mono-oxide), and so forth) is present so that NOx can be purified (removed from the engine. An NOx adsorption capability of NOx reduction (removing) catalyst has a limitation. Hence, before an absorption of the NOx removing catalyst has reached to a saturation quantity thereof, it is necessary to release the absorbed NOx from the NOx removing catalyst and reduce it to regenerate the NOx (reduction or removing) catalyst at an appropriate timing. Therefore, a fuel is added into the exhaust gas which is a reducing agent in a short period of time at an appropriate timing in the exhaust gas and the engine is temporarily switched into a rich drive, the air-fuel ratio of the exhaust gas is temporarily switched into a rich state, while the reducing agent (CO (Carbon mono-oxide), HC(Hydro Carbon), and so forth) is supplied. This procedure is called an execution of a rich spike control. On the other hand, to assure NOx reduction processing as described above, it is important to detect the abnormality such as deterioration of NOx (removing) reduction catalyst with a high accuracy. Hence, various methods of executing the abnormality determination of the NOx removing catalyst have been proposed.
For example, in a first previously proposed NOx removing catalyst abnormality determining apparatus disclosed in a Japanese Patent Application First Publication No. 2002-38929 published on Feb. 6, 2002, the air-fuel ratio of the exhaust gas streaming into the NOx removing (reduction) catalyst is reduced as compared with a case where the absorption capability of NOx (reduction) removing catalyst is temporarily regenerated and, thereafter, a deterioration of an absorption-storage reduction type NOx catalyst is determined on the basis of a time duration for which the air-fuel ratio streaming out of the NOx catalyst indicates a rich air-fuel ratio.
In a second previously proposed NOx reduction (removing) catalyst abnormality determining apparatus disclosed in a Japanese Patent Application First Publication No. 2001-73747 published on Mar. 31, 2001, the deterioration of the NOx reduction (removing) catalyst is determined on the basis of a measurement time from a time point at which the output of an oxygen concentration sensor located at an upstream side of the exhaust passage with respect to the NOx removing (reduction) catalyst is varied to a time point at which another oxygen concentration sensor located at a downstream side of the exhaust passage with respect to the NOx removing (reduction) catalyst when the air-fuel ratio of an air mixture fuel supplied to the engine is switched from a rich state to a lean state and another measurement time from a time point at which the output of the oxygen concentration sensor located at the upstream side of NOx removing (reduction) catalyst is varied to a time point at which the output of the oxygen concentration sensor located at the downstream side of NOx removing (reduction) catalyst to a time point at which the output of the oxygen concentration sensor located at the downstream side of NOx removing (reduction) catalyst is varied when the air-fuel ratio of the air mixture fuel supplied to the engine is switched from the rich state to the lean state.
Furthermore, in a third previously proposed NOx removing (reduction) catalyst deterioration (abnormality) determining apparatus disclosed in a Japanese Patent Application First Publication No. Heisei 11-324654 published on Nov. 26, 1999 (which corresponds to a U.S. Pat. No. 6,216,449 issued on Apr. 17, 2001), a time interval from a time at which the air-fuel ratio of the exhaust gas is switched from the lean state to the rich state to a time at which this rich exhaust gas is passed through the catalyst and another time interval from a time at which the air-fuel ratio of the exhaust gas is again switched from the rich state to the lean state and the lean exhaust gas is passed through the catalyst from the timing at which the above-described switching is performed are measured. The measured respective time intervals are used to evaluate an oxygen adsorption and storage function and NOx adsorption function and NOx adsorption function are evaluated. That is to say, in any case of the above-described three previously proposed apparatuses, the determination on the deterioration of the NOx removing catalyst is carried out using a time duration for which an exhaust gas atmosphere (a ratio between the oxidizing agent and reducing agent) streaming out of the catalyst is detected during the rich spike control at which NOx absorbed in the catalyst is released and reduced and the detected exhaust gas atmosphere falls in a predetermined range.